This invention relates to nondestructive testing and particularly to the nondestructive testing of a conductive material by utilizing microwaves to generate eddy currents in the material.
Nondestructive testing methods exhibit distinct advantages over other evaluation methods. Since the object to be tested is not destroyed, for example, an entire inventory may be tested to achieve a zero defect level. Furthermore, cleanup operations following nondestructive testing are usually minimal, such testing may frequently be done in the field, etc.
In nondestructive test methods utilizing the eddy current inspection technique, a search coil is used to generate an eddy current distribution within the electrically conducting part to be tested. The electromagnetic field generated by such eddy currents affects the electrical and magnetic characteristics of the coil, and these changes may be measured and used to determine the condition of the part.
Typical eddy current inspection systems have been restricted to the relatively low frequency portion of the electromagnetic spectrum, the frequencies generated in such systems ranging from approximately 2 KHz to 2 MHz. Ferrite cores may be employed in the prior art search coils to concentrate the flux to improve spatial resolution, and frequencies above 1 MHz may be used where the detection of small surface flaws is necessary, so that the depth of penetration (skin depth) is reduced and the interaction of the eddy currents with surface flaws is increased. In spite of these refinements, however, the ultimate detection capability of conventional coils is limited by considerations of physical size and frequency capacity. Further development of such coils appears to be hampered by the difficulty of fabricating very small search coils and by the unavailability of ferrite materials which will achieve adequate flux concentrations with high permeability at high frequencies.
An alternative approach to high frequency eddy current testing has been made feasible by substituting a ferromagnetic resonator for the standard search coil. Such a crystal will resonate when placed in a static magnetic field and subjected to a changing electromagnetic field at microwave frequencies due to electron spin procession. Ferromagnetic probes are inherently more compact than conventional high frequency eddy current probes and, in addition, have a potentially high sensitivity due to the high Q of the resonance phenomenon.
Conventional eddy current measurements are typically performed by sensing the input impedance of the search coil. In an analogous manner, a ferromagnetic resonator is connected in the reflecting mode and changes in its reflection coefficient are measured at resonance. It would be useful, however, to independently measure the real and imaginary parts of the impedance, so that both the phase shift and the change in magnitude of the impedance due to a flaw might be detected. These quantities are separately useful in the characterization of flaws in the material. Single port resonator probes, however, are not amenable to accurate phase measurement since such a probe must be operated in the reflecting mode and thus absorbs a maximum amount of energy at resonance.
Therefore, a need has developed in the eddy current nondestructive testing field for an apparatus and method which will permit the measurement of changes in both the magnitude and phase of a signal in a ferromagnetic resonant circuit at microwave frequencies.